Star Trek Canon Database

WORF: There is another possibility. It could be the real Kahless. He may have actually returned, as he promised.

DATA: The appearance of Kahless in the lava cave is consistent with the stories found in the Klingon sacred texts.

RIKER: No offense, Worf... but I have trouble believing that the man I just escorted to deck eight is supernatural.

WORF: I am not saying that he is. I merely think we should not completely reject the possibility.

PICARD: We have no reason to rule out anything.

Culture: Klingon culture appears to be dominated by a single unified religion. Not only is the belief in (and interpretation of) religious stories and texts virtually uniform across their society, but even Worf, a Klingon raised in isolation, seems to accept this dogma once exposed to it.

This is quite interesting; Klingons appear to have banished any secular movement from their society! Their culture and their unified religion appear to be indivisible. Even their government is beholden to their religious system, as evidenced later in this episode when Gowron is forced to appoint the clone Kahlass as Emperor. In other words, there is no separation of church and state, and apparently no religious diversity or freedom.

A society structured thusly cannot possibly thrive in a technological era. It is not a coincidence that the rise of science coincided with the rise of the secular state (more than half of the scientists throughout all of human history lived during the 20th century). The development of science and technology require a strong rational mindset and an environment open to revolutionary ideas, not a mindless adherence to dogma and an environment hostile to diversity.

Since Klingon society seemed to be more progressive in Kirk's time, it is possible that the Klingon Empire may be headed into an age of darkness much like the Dark Ages on Earth, when priceless cultural artifacts and libraries were obliterated by Christians bent on eliminating intellectual opposition (for example, the Byzantine libraries were completely destroyed, and many ancient philosophical and mathematical texts were defaced and overwritten with prayers). Given enough time, this kind of society will either undergo a secular revolution or it suffer the fate of China during the industrial revolution, and it will fall behind its competitors (unless they are also stunted in their technological growth, and that's a distinct possibility given the highly centralized nature of scientific research in the Federation, the Romulan Empire, and the Dominion).